1. Field of the Invention
The present invention relates to a gas sensor element for detecting a specific gas concentration in a measured gas (target gas) and a gas sensor using the same.
2. Description of Related Art
In an exhaust system of an internal combustion engine and the like for a vehicle, a gas sensor is disposed to detect a specific gas concentration (oxygen concentration, for example) in a target gas, such as an exhaust gas (see Japanese Patent Application Publication No. 08-240559 (JP 08-240559 A), for example). Such a gas sensor houses a gas sensor element that has, for example, an oxygen ion conductive solid electrolyte body, a measuring electrode and a reference electrode that are respectively provided in one surface and the other surface of the solid electrolyte body, and a diffusion layer that covers the measuring electrode and allows the target gas to permeate therethrough.
Among conventional gas sensor elements, the element is configured such that an outer surface thereof comes into contact with the exhaust gas. However, water vapor contained in the exhaust gas is condensed and becomes water drops when the internal combustion engine is started, and there is a case where the water drops are splashed together with the exhaust gas on the element. Here, the gas sensor element is used under a heated condition at a high temperature so that the solid electrolyte body is activated. Accordingly, a significant thermal shock is applied to the element due to adhesion of the water drops, which possibly causes water-induced cracking. In addition, a poisoning substance that adversely affects sensing performance may be contained in the exhaust gas. Therefore, JP 08-240559 A and Japanese Patent Application Publication No. 2012-93330 (JP 2012-93330 A) disclose an oxygen concentration detector in which a surface protective layer with water repellency is provided on an outer side of the element and porous layers are laminated to prevent heat transfer and to catch the poisoning substance.
Although techniques in the above documents presuppose use of the protective layer with water repellency, there is a possibility that the water repellency cannot be maintained sufficiently with time. In other words, when constituent particles of the surface protective layer are coated with the poisoning substance (a particulate oxide, for example) that is contained in the exhaust gas, the water repellency thereof is possibly degraded. Furthermore, the function of the porous layer to catch the poisoning substance becomes ineffective for the poisoning substance that is dissolved in a liquid (water, for example).
Fuel is basically made of hydrocarbons but also contains various impurities such as nitrides, water, mineral elements, and metallic elements derived from an additive. These impurities turn into a composite/mixed compound (the poisoning substance) that adversely affects defecting performance of the gas sensor and that generally exists in the exhaust gas. The above poisoning substance and water form a complex system in an exhaust system of the internal combustion engine due to factors such as the structure, combustion control, and a property of the fuel. In order to solve the above, plural inventions have reported improvement of the protective layer by taking wetness and poisoning into account. However, a sufficient effect has not been achieved.